Via.hs

{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Via where

import Data.Kind (Type)
import Data.Semigroup (Last)
import Data.Functor.Compose (Compose)

newtype Foo (a :: Type) = Foo (Maybe (Last a))
  -- This can be rewritten as 'deriving newtype (Semigroup, Monoid)'
  deriving (Semigroup, Monoid) via (Maybe (Last a))
  -- This can't
  deriving (Functor) via (Compose Maybe Last)
  
newtype Bar (a :: Foo) = Foo (Maybe (First a))
  deriving (Semigroup, Monoid) via (Maybe (First a))
  
-- If I were to change Bar's representation away from 'Maybe (First a)', I would get an error from the via derivation.
-- This reminds me to check the semantics, make sure it's still doing what I want, etc.
-- If I had used a newtype derivation, this would be _silently_ allowed.